1. Field of the Invention
The instant disclosure relates to a fan system; in particular, to a fan system, a single-phase DC motor control circuit and control method thereof able to smoothing current of the single-phase DC motor.
2. Description of Related Art
A single-phase DC motor has been widely used in many electronic products, such as a personal computer, an electric shaver, a copier, a projector a blender and other kind of low voltage electronic products. With the advancement of technology, demand for personal computer with high performance is also increased continuously. For a personal computer with high performance, temperature of a host is easy to rise because increase of the operation speed central processing unit in the unit time. Therefore, the personal computer of new generation needs to use fan with the single-phase DC motor for cooling, and thus fan with the single-phase DC motor plays a decisive role in the miniature heat-dissipation fan system. For the above reasons, it is important to provide a circuit and control method thereof for stabilizing operation of the single-phase DC motor.
Referring to FIGS. 1 and 2, FIG. 1 shows circuit block schematic view of the control circuit of the single-phase DC motor in the prior art. FIG. 2 shows waveform schematic view of the output signal in FIG. 1. The control circuit of the single-phase DC motor senses a magnetic pole position of an inner rotator of the single-phase DC motor 130 through utilizing a hall element HAL, and accordingly generate a first sinusoidal wave signal BOP and a second sinusoidal wave signal BRP. After the first sinusoidal wave signal BOP and the second sinusoidal wave signal BRP are transmitted to the comparator RP1 and RP2, the comparator RP1 and RP2 respectively outputs hall signals HC1 and HC2 according to a comparison operation. Next, the logic circuit 112 receives a pulse modulation signal PW generated from the external PWM generator 120 and the hall signals HC1 and HC2 so as to respectively output the switch signal H1,H2,L1 and L2 for controlling switched-on or switched-off state of each switch unit (not shown) in the driving circuit 114. Afterwards, the driving circuit 114 alternatively outputs the first output signal VOUT1 and the second output signal VOUT2 to the single-phase DC motor 130 for making the single-phase DC motor 130 rotate.
However, when the single-phase DC motor 130 rotates, a BEMF must be generated in the single-phase DC motor 130, wherein polarity of the BEMF is opposite to that of the applied voltage. Therefore, a current flowing through the single-phase DC motor 130 may be affected by the BEMF, so that a current flowing through the single-phase DC motor 130 may generate change correspondingly, e.g. waveform of saddle-type current shown in FIG. 2. Accordingly, in the phase-changing duration of outputting the first output signal VOUT1 and the second output signal VOUT2 (i.e. voltage level of the hall signals HC1 and HC2 are low voltage level), the current flowing through the single-phase DC motor 130 may be change sharply (e.g. higher tail current value of the saddle-type current). Current of sharply changing will affect operation of the single-phase DC motor 130, so as to generate mechanical noise when the single-phase DC motor 130 rotates